In so-called fly-by-wire aircraft, aircraft control surfaces are not linked to the controls by mechanical means. Rather, the linking is via electrical or hydraulic circuits. Consequently, in the event of an electrical or hydraulic failure, the aerodynamic configuration of the aircraft cannot be altered under the control of the pilot until power is restored. As a result, such aircraft require an emergency power unit which is capable of responding to a power failure and providing a sizable quantity of electrical or hydraulic energy in very short order so that control of the aircraft can be returned to the pilot.
Fly-by-wire aircraft, like other aircraft of more than basic simplicity, also require an auxiliary power unit for providing electrical and hydraulic energy and bleed air when the main engine or engines of the aircraft are not in use. Quite typically, both an emergency power unit and an auxiliary power unit will each employ a gas turbine engine coupled to a generator and a hydraulic pump. Thus, where an aircraft employs an emergency power unit and an auxiliary power unit, there will be two systems with a substantial degree of redundancy. This of course requires a certain space on the aircraft and will cause some weight concerns.
While in some aircraft an auxiliary power unit may be easily adapted to serve as an emergency power unit as well and thereby minimize space requirements as well as weight concerns, the adaptation is not so simple on high performance aircraft that may operate at rather high altitudes. In particular, because a typical auxiliary power unit turbine is an air breathing turbine, at high altitudes the density of the air will be insufficient to start the turbine and rapidly bring the same up to a speed at which it will operate at that altitude to produce emergency power in an emergency situation.
To meet these and other problems, Friedrich, in his U.S. Pat. No. 4,092,824, issued June 6, 1978, proposes a turbine for use in aircraft for starting purposes as well as for driving auxiliary equipment such as a generator and which is capable of operating in a conventional air breathing mode as well as in an emergency mode that does not require the presence of air. In particular, Friedrich includes a supply of hydrazine on the aircraft. Hydrazine is capable of undergoing an exothermic decomposition reaction and the heat from this reaction is utilized by Friedrich to vaporize aircraft fuel to thereby provide a volume of hot gas to drive the turbine in an emergency situation.
While this solution does solve a number of the previously specified problems, it also creates some new ones. In particular, the decomposition products of hydrazine can accumulate much like soot within the turbine and that will decrease turbine efficiency when operating conventionally. Perhaps more significantly, because the basis of such system is that of an exothermic decomposition reaction, it necessarily follows that a fuel such as hydrazine, which is utilized in the system, must be sufficiently unstable as to rapidly undergo decomposition. Of course, the presence of a fuel that is not stable in the conventional sense on an aircraft presents hazards of its own.
Still another difficulty resides in the fact that hydrazine and proper storage facilities therefore may not be available at all ground support locations for the aircraft. Thus, the servicing of a system whose hydrazine fuel charge has been partially or wholly consumed becomes a problem, particularly since hydrazine is toxic and is not easily handled.
Consequently, there is a need for a combined auxiliary power unit and emergency power unit system which can be easily serviced to minimize logistic difficulties at ground support locations, both from the standpoint of equipment necessary to support the system and from the standpoint of ease of handling. The present invention is directed to solving these problems.